Phase 7: Replace render_template() with s-expression rendering in all POST/PUT/DELETE routes

Eliminates all render_template() calls from POST/PUT/DELETE handlers across
all 7 services. Moves sexp_components.py into sexp/ packages per service.

- Blog: like toggle, snippets, cache clear, features/sumup/entry panels,
  create/delete market, WYSIWYG editor panel (render_editor_panel)
- Federation: like/unlike/boost/unboost, follow/unfollow, actor card,
  interaction buttons
- Events: ticket widget, checkin, confirm/decline/provisional, tickets
  config, posts CRUD, description edit/save, calendar/slot/ticket_type
  CRUD, payments, buy tickets, day main panel, entry page
- Market: like toggle, cart add response
- Account: newsletter toggle
- Cart: checkout error pages (3 handlers)
- Orders: checkout error page (1 handler)

Remaining render_template() calls are exclusively in GET handlers and
internal services (email templates, fragment endpoints).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

federation/app.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 import path_setup  # noqa: F401  # adds shared/ to sys.path
-import sexp_components  # noqa: F401  # ensure Hypercorn --reload watches this file
+import sexp.sexp_components as sexp_components  # noqa: F401  # ensure Hypercorn --reload watches this file
 from pathlib import Path
 
 from quart import g, request
@@ -96,7 +96,7 @@ def create_app() -> "Quart":
     async def home():
         from quart import make_response
         from shared.sexp.page import get_template_context
-        from sexp_components import render_federation_home
+        from sexp.sexp_components import render_federation_home
 
         ctx = await get_template_context()
         html = await render_federation_home(ctx)